The present invention relates generally to pulse width modulated (PWM) amplifiers and more specifically to a servoamplifier for a DC electric servomotor.
To obtain optimum performance from a DC servomotor it should see current waveforms that contain only frequencies present in the command signal or load. Any additional AC signals especially those generated by the controller reduce the motor's maximum capability by generating extra heat within it. The heat is in the form of extra copper losses and extra iron losses. Copper losses resulting from extra AC components can be qualified by the form factor, i.e., the ratio of the RMS current to the average or torque producing current. Iron losses are more difficult to quantify since there are two varieties and their values do not always vary linearly with current or frequency. Iron losses, however, always increase with frequency and current. Conventional SCR amplifiers generally have poor form factors but inject only low frequency ripple components. Thus, the extra heating associated with SCR amplifiers is mainly in the form of copper losses.
PWM amplifiers, on the other hand, do tend to have better form factors but also operate at much higher frequencies. This tends to cause higher iron losses rather than higher copper losses. These losses are tolerated however, because the PWM amplifier provides greatly improved system bandwidth, better large signal transient response and tends to induce fewer machine reasonance problems.
A representative example of a pulse width modulated amplifier is disclosed in U.S. Pat. No. 4,368,411, entitled "Control System For Electric Motor", by H. Keith Kidd, issued Jan. 11, 1983. The Kidd patent discloses a four quadrant control system for a brushless DC motor wherein the windings are energized by pulse width modulated control signals applied to a switching circuit in a bridge configuration. A pulse width modulator provides the control signals in response to a comparison of a pair of triangular waves to an error signal. The pulse width modulator is capable of providing relatively high currents having good form factors to motor, but it is unable to provide a "dither" current.
Due to the relationship between the form factor and the magnitude of the average load current, it has often been found that better overall machine performance results when some AC signal is fed to the motor at standstill. This "dither" component prevents the higher stiction forces from coming into play yet does not appreciably increase overall motor heating since it occurs at low average current levels. Thus, it is desirable for a servoamplifier to have the capability of injecting a controlled amount of "dither" current into the motor at no load or light loads but to smoothly leave this mode and progress to a type of operation where the current contains no extra frequencies generated by the amplifier as the current becomes higher.
Copending U.S. patent application Ser. No. 472,216 entitled "Motor Control System With Load Current Recirculation", filed Mar. 4, 1983 discloses a bidirectional three-phase SCR motor control system which can provide a "dither" current at standstill. Such a three-phase SCR servoamplifier however, as mentioned above, has an inherently lower bandwidth, although it does provide the desirable decreasing form factors at higher current values.
Accordingly, there is a need for a servoamplifier which can provide the advantages of a recirculating load current SCR servoamplifier with the high bandwidth advantages of a PWM servoamplifier.